Method of controlling slow scan direction of spot position in an optical output device employing an electro-optical control apparatus

ABSTRACT

A method of slow, or process, scan direction spot registration or position control in an optical output device, such as a raster output scanner (ROS), may be facilitated by interposing in the image path an electro-optic element whose angular dispersion varies for a given wavelength as a function of the electrical bias applied to it. By orienting the electro-optic element such that dispersion control is perpendicular to the fast or line scan direction of the ROS, varying the electrical bias applied to it varies the dispersion in the slow scan direction. The electro-optic element may be, for example, a prism of AlGaAs. Bias applied to the electro-optic element may be in response to the output of a means for detecting and quantifying such positional errors and/or in response to predetermined correction information output from a processor controlled memory unit or the like. Spot position for single or multiple beam optical output device may be achieved.

BACKGROUND OF THE INVENTION

The present invention relates generally to optically output devices, andmore specifically to a method for providing position or registrationcontrol of a spot or spots at which a light beam strikes aphotoreceptive element, which is achieved by varying the index ofrefraction, diffraction, etc. of an electro-optic element located in thelight beam's path.

The present application relates to the following concurrently filed U.S.patent applications: Ser. No. 07/747,039; Ser. No. 07/747,166; and Ser.No. 07/747,889. Each of these applications is hereby incorporated byreference.

Although applicable to a wide variety of optical output devices, thepresent invention finds particular utility in Raster Output Scanning(ROS) apparatus. Therefore, the following details and descriptions beginwith a background of the present invention in terms of ROS apparatus.Ros has become the predominant method for imparting modulated lightinformation onto the photoreceptor in printing apparatus used, forexample, in digital printing, and has found some application in otherimage forming operations such as writing to a display, to photographicfilm, etc. Consider, for illustration purposes, what is perhaps the mostcommon application of ROS, digital printing. As is known, the scanningaspect thereof is conventionally carried out by a moving reflectivesurface, which is typically a multifaceted polygon with one or morefacets being mirrors. The polygon is rotated about an axis while anintensity-modulated light beam, typically laser light, is brought tobear on the rotating polygon at a predetermined angle. The light beam isreflected by a facet and thereafter focussed to a "spot" on aphotosensitive recording medium. The rotation of the polygon causes thespot to scan linearly across the photosensitive medium in a fast scan(i.e., line scan) direction. Meanwhile, the photosensitive medium isadvanced relatively more slowly than the rate of the fast scan in a slowscan direction which is orthogonal to the fast scan direction. In thisway, the beam scans the recording medium in a raster scanning pattern.The light beam is intensity-modulated in accordance with a serial datastream at a rate such that individual picture elements ("pixels") of theimage represented by the data stream are exposed on the photosensitivemedium to form a latent image, which is then transferred to anappropriate image receiving medium such as sheet paper.

Although, for the purposes of example, this discussion is in terms ofROS apparatus, it will be apparent from the following discussion thatthere exists many other scanning and non-scanning system embodiments ofthe present invention. However, as a convention, the word "scan" will beused when referring to the fast and slow scan directions (i.e., motionor position in the fast and slow scan planes), with the understandingthat actual scanning of the spot is not absolutely required.

Data in each of the the fast and slow scan directions is generallysampled. The sampling rate of the slow scan direction data equates to300 lines per inch or more in many printing apparatus. It has been shownthat errors in the slow scan direction of as small as 1% of the nominalline spacing may be perceived in a half tone or continuous tone image.This implies a need for a high degree of spot position control in theslow scan direction on the image plane, especially in such applicationsas multiple beam and multiple ROS color printers where control of theposition of multiple spots is critical. Furthermore, high resolutionprinting, on the order of 600 spots per inch or higher demands veryaccurate spot positioning.

Errors of the spot position in the slow scan direction arise from manysources, including polygon and/or photosensitive medium motion flaws,facet and/or image plane (e.g., photosensitive medium) surface defects,etc. These errors are most commonly addressed by passive or activein-line optics. Positional errors which extend over an entire scan lineare most commonly compensated for by retarding or advancing the start ofscan by one or more scan lines (this correction being limited to wholemultiples of a scan line spacing). See, for example, Advances in Laserand E-O Printing Technology, Sprague et al., Laser Focus/Electro-Optics,pp. 101-109, October 1983. Another approach employing passive optics isthe use of extremely high quality optical and mechanical elements. Thisnecessarily implies higher overall costs, and possible limitations onthe durability of the system. Still another example of passive opticalcorrection is the system disclosed in U.S. Pat. No. 4,040,096, issuedAug. 2, 1977 to Starkweather, which accommodates a basic polygon ROSstructure having runout and/or facets errors (both scanning errors inthe slow scan direction) by locating a first cylindrical lens in thepre-polygon optical path, which focuses the beam in the slow scandirection onto the facet, and a second cylindrical lens in thepost-polygon path, which focuses the facet onto the desired image plane.Toroidal elements and concave mirrors have also been to used toaccomplish the same function.

Active compensation for process scan direction errors usually involves aclosed loop and/or memory-fed compensation system. A closed loopacousto-optical (A-O) compensation system is discussed in Laser Scanningfor Electronic Printing, Urbach et al., Proceedings of the IEEE, vol.70, No. 6, June 1982, page 612, and the reference cited therein. Asdiscussed in this reference, slow scan spot position detector is placedin the scan line which, together with related processing apparatus, iscapable of quantifying the slow scan displacement. An A-O element isdisposed in the optical path whose refractive index may be varied byestablishing therein an acoustic wave. A variation in the acoustic wavegenerated in the A-O element is accompanied by a variation in thedispersion angle (that is, the angle of the output beam relative to theangle of the input beam). The slow scan displacement information fromthe detector and processing apparatus is fed to the acoustic wavegenerating portion of the A-O device, which may then control the slowscan direction position of the scan line in response to the displacementinformation. Further, the control information for certain recurrentdisplacement errors may be measured in advance and synchronized with theangular motion of the rotating polygon, as discussed in the abovereference. See also Visibility and Correction of Periodic InterferenceStructures In Line-by-Line Recording Images, J. Appl. Phot. Eng., vol.2, pp 86-92 Spring 1976.

One technology which, although it is directed to a method of scanning,as opposed to addressing slow scan direction errors, it nonethelessrelevant is disclosed in Fast Dispersive Beam Deflectors and Modulators,Filinski and Skettrup, IEEE Journal of Quantum Electronics, vol. QE-18,no. 7, pp 1059-1062, July 1982. As briefly described therein, a staticoptical element having dispersion characteristics which vary as afunction of the wavelength of the incident light can be utilized to scanin one dimension by varying the output wavelength of the light source.Various types of static dispersive elements are mentioned thereinincluding prisms and gratings, although no details about incorporationof this type of scanning element into a complete scanning system areprovided. Nor is there any mention in that reference about employing thedescribed apparatus to control slow scan direction spot position.

There is presently a need in the art for spot position control apparatusand methods which provide improved continuous, very high resolutiondeflection of an optical beam in the slow scan direction.

Shortcomings of spot position control schemes known in the art includethe complexity, cost and/or the difficulty of manufacture of suchsystems. For example, the use of high quality optics requires not onlyhigh quality optical elements, but utmost control in the positioning ofthose optics in order to obtain the requisite very precise mechanicalcontrol sufficient to adjust spot position 0.02 mm or less, required inmany cases. In order to achieve this level of spot position control withthe aforementioned acousto-optic modulators, an acoustic wave must beestablished and maintained with great precision. These acousto-opticmodulators are relatively quite expensive, and require an associatedaccurate high frequency signal generator and related electronics toproduce and maintain the acoustic waves.

Two further disadvantages of many prior art spot position controlschemes are the speed and precision at which they are capable ofoperating. For example, three of the most common ROS schemes, cylinderlenses, rotating mirrors, and translating roof mirrors are generally tooslow to correct for motion quality errors or line-to-line errors, whilerotating mirrors and translating roof mirrors are also large andtherefore quickly difficult to move precisely and quickly.

SUMMARY OF THE INVENTION

The present invention provides a novel method for controlling the spotposition or registration in the slow scan direction in an optical outputsystem which overcomes a number of problems and shortcomings of theprior art. Spot position refers to the location that a light beam isincident upon an image plane, and spot registration refers to thelocation that the light beam is incident on that image plane relative toother spot positions (for example in overwriting a spot for tone,position, color, or control of other parameters). However, forsimplicity of explanation, any reference to control of spot positionwill include control of spot registration, unless otherwise noted. Ingeneral, the spot position control is provided by interposing in theimage path an electro-optic element whose angular dispersion varies fora given wavelength as a function of the electrical bias applied to it.Spot position control is achieved by controlling the electrical biasapplied to the electro-optic element. Spot position control may beachieved for either a single spot or for multiple spots where theposition of each spot relative to the other spots is maintained.Depending on the output parameters of the optical output apparatusembodying the present invention, spot position control may be achievedon a pixel-by-pixel basis.

One embodiment of the method of the present invention employs a rasteroutput scanning apparatus of the type including a light source,typically a laser light source, for emitting a light beam, means formodulating the light beam in accordance with a data signal, means forscanning the light beam in a raster fashion, and image plane means(e.g., a photoreceptive element, display screen, photoreceptive film,etc.) for receiving the scanned light beam. Included in theaforementioned ROS apparatus is an electro-optic means for controllingthe slow scan direction position of the light beam at the point that itis incident upon the image plane means, disposed in the optical pathbetween the light source and the image plane means. Means fordetermining the existence and extent of spot position errors and/or theneed for application of predetermined spot position correction may alsobe included.

In operation, a light beam is generated by the light source which ismodulated in response to an image data signal. The light beam is scannedacross at least a portion of a surface of the image plane means in afast scan plane, as well as scanned across at least a portion of asurface of the image plane means in a slow scan plane which is normal tothe fast scan plane. The existence and extent of error, if any, in theposition of the light beam in the slow scan plane is determined for apart or all of the scan in the fast scan plane, and correction for anyslow scan plane error is performed by varying the the opticalproperties, specifically the index of refraction, of the electro-opticelement through which the light beam passes by varying its electricalstate (or optical properties) in response to the determination of theexistence and extent of such error.

The control of spot position on the image plane means provided by thepresent invention may be employed to correct for interline slow scanplane position al errors by varying the electrical bias applied to theelectro-optic element in response to the output of a means for detectingand quantifying such positional errors and/or in response topredetermined correction information output from a processor controlledmemory unit or the like.

Furthermore, the maximum amount of slow scan plane spot positioncorrection that will be required will be equal to one half of a scanline height. Any greater amount of correction may be realized through acombination of the above spot position control and retardation oradvancement of one or more scan lines.

The scope of the present invention and the manner in which it addressesthe problems associated with prior art methods and apparatus will becomemore readily apparent from the following detailed description when takenin conjunction with the accompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side or elevation view of the general opticalconfiguration of an apparatus functioning in accordance with oneembodiment of the present invention, showing an electro-optic element inthe form of a prism disposed between the light source and the rotatingpolygon scanning device of a ROS system.

FIG. 1A shows a photoreceptive drum at the image plane of the apparatusof FIG. 1 as might be employed in a xerographic printing application ofthe present invention.

FIG. 2 shows a top or plan view of the general optical configuration ofthe apparatus of FIG. 1, showing an electro-optic element disposedbetween the light source and the rotating polygon scanning device of aROS system.

FIG. 3 shows a side or elevation view of the general opticalconfiguration of another embodiment of the present invention, showing anelectro-optic element disposed between a multiple-beam light source anda rotating polygon scanning device of a multiple beam ROS system.

FIG. 4 shows in detail an electro-optic element which may be employed inthe present invention to allow controllable spot registration in a ROSsystem.

FIGS. 5 and 5A show a schematic representation of ROS system for thepurposes of describing the nature and extent of the control of spotposition provided in the process scan direction by the presentinvention.

FIG. 6 shows a side or elevation view of the general opticalconfiguration of an apparatus according to the first embodiment o thepresent invention, showing a means for detecting errors in the positionof the photoreceptive drum and feeding a measure of the detected errorand predetermined correction data back to the electro-optic element as acontrol signal for adjusting the position of the laser beam emergingfrom the electro-optic element.

FIG. 7 is a flow diagram of one embodiment of the method of the presentinvention for determining and correcting for slow scan direction errorson the fly, and for compensating for predetermined slow scan directionspot position errors.

FIG. 8 is a side or elevation view of a non-scanning embodiment of thepresent invention.

In general, like reference numerals will be used to denote like elementsas between each of the aforementioned figures.

DETAILED DESCRIPTION

A detailed description of a first embodiment of the present invention ispresented herein with reference to FIGS. 1 and 2, which show,respectively, slow scan plane and fast scan plane views of a scanningapparatus 10. Apparatus 10 is a raster output scanning device of thetype which may, for example, output a scanned and modulated outputsignal to a photoreceptive drum 12, such as that shown in FIG. 1A, foruse in a xerographic printing process. Alternatively, apparatus 10 mayoutput a scanned and modulated optical signal to a display device,photographic device or other application employing such a scanned andmodulated optical signal.

Apparatus 10 includes a light source 14, such as a solid state laser orarray of lasers, which produces a diverging beam of coherent light 16.In the path of beam 16 are first cylindrical lens 18, which has power inthe fast scan plane only, second cylindrical lens 20, which has poweronly in the slow scan plane, electro-optical device 22, which isdescribed in further detail below, third cylindrical lens 24, which haspower in the fast scan plane only, scanning device 26, which is shown asa rotating polygon having at least one reflective facet 28 (but whichmay also be a rotating hologram, rotating diffraction grating, etc.),spherical lens 30, which has power in both the fast and slow scanplanes, and toroidal lens 32. The path of beam 16 terminates at imageplane 34, which may be a line on the aforementioned rotatingphotoreceptive drum 12 (FIG. 1A), a surface of a ground glass or othertype of display screen, a photosensitive film, etc.

Referring to FIG. 2, which shows the fast scan plane view of apparatus10, the diverging beam of light 16 emitted by source 14 is focused byfirst cylindrical lens 18 onto the entrance aperture of electro-opticdevice 22, through cylindrical lens 20. The electro-optic device 22 isvery narrow in the fast scan plane, causing it to behave as a onedimensional optical waveguide. (See FIG. 3 and the accompanyingdescription below.) Because the electro-optic device 22 behaves as a onedimensional optical waveguide in the scan direction, the light focussedon its entrance aperture propagates through it, and diverges as itleaves its exit aperture. Cylindrical lens 24 then collimates the lightin the scan plane prior to its arrival at facet 28 of scanning device26.

Electro-optic device 22 is shown in the apparatus of FIGS. 1 and 2 as aprism. However, electro-optic device 22 may be one of a variety ofdevices and material compositions discussed in greater detail below.

Although modulation of the beam may be conveniently achieved by directlymodulating the output of the light source, for example by modulating thecurrent applied to the laser itself from below to above the lasingthreshold, as known in the art, the beam may be projected to a modulator(not shown) which may be one of any number of types of modulators, suchas an electro-optic or acousto-optic modulator, TIR modulator, etc.

The beam is next incident upon a scanning device 26, which may be one ofa variety of such devices known in the art, most typically a rotatingpolygon with a least one mirrored facet 28. Other suitable devices forscanning include rotating holograms, and rotating diffraction gratings,etc. As shown most clearly in FIG. 2, the rotation of the mirroredfacet(s) causes the beam to be deflected and thereby scanned across animage plane 34.

Beam 16, having been appropriately deflected (i.e., reflected) byscanning device 26, diverges, and lenses 30 and 32 are employed torefocus the beam to a circular or elliptical cross-section onto imageplane 34, and to correct for scan nonlinearity (f-theta correction).Toroidal lens 32, or an equivalent thereto (such as a cylindricalmirror) corrects for wobble (scanner motion or facet errors). Imageplane 34 may be ground glass, a viewing screen, a photosensitivematerial (film, electrostatic photoreceptor, etc.), or other image planeviewing or receiving medium. FIG. 1A shows the image plane 34 as a lineon a rotating photoreceptive drum 12 used in printing applications suchas xerographic printing and the like.

Thus, if scanning device 26 is rotated in a clockwise fashion, as shownin FIG. 2, a beam reflected from one of its moving facets will be causedto scan across the image plane 34, as indicated by the arrow. If thebeam may then be controllably modulated, a scanned and modulated singlebeam of general application results. If image plane 34 comprises theline on rotating photoreceptive drum 12 of FIG. 1A, and the rotation ofdrum 12 and the modulation and scanning of the beam are properlycoordinated, a ROS printer device may be realized.

An embodiment of the present invention allowing simultaneous spotposition control for a plurality of spots is shown in FIG. 3. Apparatus50 includes multiple light sources 52a, 52b, such as independent solidstate lasers, or a monolithic multiple beam solid state laser, whichproduce diverging beams of coherent light. The wavelengths of the beamswill be nearly the same, on the order of a few nm apart, in order thattheir positions relative to one another do not change during the spotpositioning process. As viewed in the fast scan plane, beams 16a and 16bpass through first cylindrical lens 18, second cylinder lens 20,electro-optic device 22, and third cylindrical lens 24, are reflectedoff facet 28, and pass through spherical lens 30 and toroidal lens 32prior to striking image plane 34, as previously described. Althoughmodulation of the beams may be conveniently achieved by directlymodulating the output of each light source, for example by modulatingthe current applied to the laser itself from below to above the lastingthreshold, the beams may be projected to a modulator (not shown), whichmay be one of any number of types of modulators, such as anelectro-optic or acousto-optic modulator, TIR modulator, etc. As before,image plane 34 may be ground glass, a viewing screen, a photosensitivematerial (film, electrostatic photoreceptor, etc.), or other image planeviewing or receiving medium.

Due to the similarly between the structure and operation of theembodiment of FIGS. 1 and 2 and the embodiment of FIG. 3, the remainderof the description of the present invention shall be with regard to asingle beam embodiment (that shown in FIGS. 1 and 2) for clarity andsimplicity. The discussion is, however, equally applicable to multiplebeam apparatus, as will be appreciated by those skilled in the art.Furthermore, many of the details of the lenses and other optical andmechanical components of a complete ROS system may be omitted forclarity since they are well known in the art.

For purposes of the following explanation, it will be assumed thatelectrooptic device 22 takes the form of an isosceles triangular prism,as shown in the FIGS. (The material composition of device 22 isdescribed in detail below.) Also, it will be appreciated that optimalresults are achieved when the electro-optic device 22, in the case thatit takes the form of a prism, is fully illuminated (i.e., fully filledwith light). This is because the resolving power of the prism isinversely proportional to the width of the optical beam, which sets alower limit on the height of the prism. However, to minimize inputelectrical power it is desirable to make the prism as small as possible.Hence fully illuminating the prism maximally utilizes its active area.However, it will be appreciated that electro-optic device 22 may takeother forms such as a diffraction grating, thin film or similar elementwhere appropriate.

Electro-optic device 22 facilitates the control of the spot positionwhich forms a basis for the present invention. This control is based onthe aspect of device 22 that its refractive index may be easily,quickly, and accurately varied. In particular, a class of such devicesexists whose index of refraction may be varied by the application of anelectrical bias to the device. For instance, the semiconductor AlGaAsexhibits this feature, as well documented in the art. In the embodimentshown in FIGS. 1 and 2, electro-optic device 22 is shown as a prism,which for the purposes of the following discussion shall be assumed tobe AlGaAs, and which is shown in more detail in FIG. 4. However, device22 may be another suitable material such as lithium niobate, liquidcrystal, etc.

Device 22 shown in FIG. 4 includes a substrate 60 having depositedthereon an n-AlyGa.sub.(1-y) As cladding layer 62, where y mighttypically be equal to 0.40, for example by MOCVD methods well known inthe art. A waveguide core 64 of Al_(x) Ga.sub.(1-x) As, whereby y>x andx might be 0.2, is next deposited on layer 62 (although waveguide coremight also be a multiple quantum well structure, where appropriate).Waveguide core 64 is chosen to have a wide bandgap so that it istransparent at the wavelength of operation. A p-Al_(z) Ga.sub.(1-x) Ascladding layer 66, where z>x and z typically is equal to y, is thendeposited on layer 64. The structure is then etched down to thesubstrate by methods known in the art to form etched facet 68. Then,three sides of the structure are cleaved to form cleaved facets 70, 72,and 74. Metallic electrical contacts 76 and 78 are next applied abovelayer 66 and below substrate 60, respectively. Anti-reflective (AR)coatings 80 and 82 are next applied to facets 68 and 74, respectively. Aprism is thereby formed which serves to selectively diffract lightincident thereupon as a function of the bias applied between thecontacts 76 and 78. The path of the diffracted light is shown by way ofthe exemplary double arrow in FIG. 4 labeled L.

FIGS. 5 and 5a show a schematic representation of apparatus 10 for thepurposes of describing the nature and extent of the control of spotposition provided in the slow scan plane by the present invention. Forthe purposes of simplicity of explanation, only elements necessary tothe explanation have been shown therein.

Rays of an optical beam from light source 14, as illustrated by arepresentative ray 90, will undergo an angular deviation ε uponrefraction by electro-optic device 22. The extent of that refractionwill vary as a function of the index of refraction of device 22. Thatis, if the index of refraction of device 22 is changed by an amount Δn,there will be a corresponding change in the angular deviation Δε. M.Born and E. Wolf show in Principles of Optics (5th ed. Pergamon Press,p. 180) that at minimum deviation, Δε is given by

    Δε=Δn(b/d)                             (1)

where b is the base of the prism and d is the width of the output beam.

To calculate the resolving power of the prism for a change in refractiveindex, referring again to Born and Wolf (p. 404, supra), the angulardivergence required to define one spot is λ/d, where λ is the wavelengthof the light. Then to resolve two spots by changing the refractive indexby Δn,

    Δε=λ/d                                (2)

from which is obtained

    Δn=λ/b                                        (3)

Thus, the required index change can be achieved by increasing the baseof the prism. By way of example, for a laser operating at 780 nm and aprism with b=1 mm, Δn=0.78×10⁻³, which is within reason for AlGaAs.

The physical principles yielding an adequate amount of index change inan AlGaAs prism may be either the linear or quadratic electro-opticeffect of free carrier injection. For the linear electro-optic effect,the value Δn=0.2×10⁻³ is consistent with the value calculated from theknown electro-optic coefficient of GaAs (1.6×10⁻¹⁰ cm/V) and 15 voltsdropped across a 2 μm layer. See, for example, I. P. Kaminow, IEEE J.Quant. Electron., vol. 4, p 23 (1968)). It is also consistent with thevalue obtained by Houghton in Electronic Letters, vol. 20, p. 479 (1984)for Al₀.25 Ga₀.75 As at 1.15 μm, and about half of the value obtained bySimes, et al., in Appl. Physics Letters, vol. 53, p. 637 (1988) for amultiple quantum well structure operated at 25 volts. Using this value,for two resolvable spots the prism base must be 3.9 mm.

A free carrier plasma will introduce a much stronger index change thanthe linear electro-optic effect described above. Thus, by properlyforward biasing the prism, carriers may be injected into the waveguidecore 64 (FIG. 4). The index change introduced by free carriers isapproximately

    Δn=(-1.14×10.sup.-21)×N                  (4)

where N is the carrier density. Thsu, a prism with a 1 mm base operatingat 780 nm requires

    N=λ/(b×1.14×10.sup.-21)=6.8×10.sup.17 /cm.sup.3 (5)

This level of carrier injection will not introduce significant beamattenuation or additional spontaneous emission.

The angle of the prism α is determined by the width of the output beam dand the prism base b from the following relation

    sin.sup.2 α/2=b.sup.2 /(4d.sup.2 +b.sup.2 n.sup.2)   (6)

Since the index change required for a given number of resolved spotsdepends only on λ and b, we fix b and tradeoff beam width for prismangle. Table 1 indicates possible values for an Al₀.2 Ga₀.3 As waveguidecore, with n=3.58, and prism base b=1 mm.

                  TABLE 1                                                         ______________________________________                                                              angle of                                                Beam width d                                                                            prism angle α                                                                       incidence φ                                                                          prism height                                 (mm)      (degrees)   (degrees)  (mm)                                         ______________________________________                                        0         32.4        87.2       1.72                                         0.5       31.2        74.3       1.79                                         1.0       28.2        60.7       2.00                                         2.0       21.5        41.9       2.63                                         3.0       16.5        30.9       3.45                                         4.0       13.1        24.1       4.35                                         ______________________________________                                    

Waveguide core 64 is of the type that allows propagation of a lightwavetherethrough while confining it in at least one dimension to the orderof one wavelength. The lightwave propagates, without diverging, in thelongitudinal direction of the guide since it is confined to thewaveguide core by the lower refractive index of the cladding layers.Confinement of the optical field of the lightwave to a thin waveguidecore is advantageous when using the electro-optic effect employed by theapparatus of the present invention since the change in the refractiveindex which the lightwave experiences has the maximum effect on thelightwave's propagation speed. Furthermore, in general, the electricfield required to produce an adequate change in the refractive index isquite high. Since the electric field in the waveguide core is given bythe applied voltage divided by the physical thickness of the core layer,a thin core such as may be employed in the above described structureallows a high electric field and hence large change in the refractiveindex. As described, 75 kV/cm (15 volts/2 μm) is required in GaAs toproduce Δn=0.2×10⁻³. If the wave could be confined to 1 μm, instead of 2μm, then either the same Δn(0.2×10⁻³) can be produced by half thevoltage (7.5 volts), or Δn may be doubled (0.4×10⁻³) for the samevoltage (15 volts).

The method of the present invention may utilize either feedback controlfor "on the fly" correction or control from stored data, or both, tomove the spot in the process scan direction to accommodate for motionquality errors, and the like, as detailed below. Initially, however,those errors must be detected. The arrangement 100 of FIG. 6 shows asimple method for determining the rotational error of a photoreceptivedrum 102 by way of a synchronized strobe and sensor arrangement 104utilizing timing marks 106 on drum 102. Arrangement 100 includesprocessing which enables determination of the existence and extent ofrotational error, and generation of a control signal in response to thedetermination of the extent of error which is transmitted to controlapparatus 108 controlling the bias applied to the electro-optic element22.

The bias for the electro-optic device 22 is supplied by a voltage source(not shown). Based on the data of Houghton in Electronics Letters, vol.20, p. 479 (1984) for a double heterostructure material, and Simes, eta., in Applied Physics Letters, vol. 53,637 (1988) for multiple quantumwell material, an operating voltage of 25 volts or less would berequired. For carrier injection devices, the operating voltage would beless than 5 volts and the operating current for a prism with a 1 mm basewould be less than 1 amp depending on the carrier lifetime of thematerial.

In addition to the above, spot position may be controlled bypredetermined correction data, which is feasible for certain recurrenterrors such as off axis rotation of a photoreceptive drum, surfacedistortion of a display screen, etc., and provision of thispredetermined correction to the apparatus controlling the bias appliedto the electro-optic element 22 from a processor controlled memorydevice 110 or the like. The output of the processor controlled memorydevice 110 could be synchronized by the strobe and control apparatus108, or other suitable synchronization arrangement

FIG. 7 details one complete cycle of operation of the method of thepresent invention for correcting for slow scan direction errors. It willbe assumed that any predetermined of required correction for recurrenterrors has been made, and that the correction data has been stored in anappropriate memory device (not shown). To begin, means (not shown) areemployed to determine whether the current scan line is one for whichpredetermined correction data has been stored. This is shown at step200. If such data exists, the data is converted into a bias signal whichis applied to the electro-optic element in order to correct forpredetermined spot position error, as shown at 202. Once the correctionfor predetermined errors has been made, or if no such predeterminederror data exists, the light beam is generated at 204. Next, theposition that the beam is incident on the image plane is determined at206 (alternatively, error in photoreceptor motion or positioncorrectable by selective spot positioning is determined). If there isslow scan direction position error at this point, the extent of thaterror is determined by appropriate determining apparatus, for example bythe aforementioned strobe and sensor arrangement. The extent of thaterror is converted to an appropriate electrical bias signal which iscommunicated to the electro-optic element at 208 in order to correct, onthe fly, for the determined error. Once the correction for this errorhas been made, or if it is determined that no such error exists, thebeam may then be scanned and modulated in order to write the scan lineat 210. When the end of scan is detected, a call is made for the nextscan line data at 212, the scan processes in the slow scan direction andthe process begins again at 200.

It will be apparent that, depending on the operating parameters of theoptical output apparatus embodying the present invention, spot positioncontrol may be achieved on a pixel-by-pixel basis. For example, considera relatively high performance laser xerographic printing apparatus withtypical operating parameters of 60 page per minute output, 600×600 spotper inch resolution, and 14 inch scan. For this device, a typical pixelexposure time is on the order of 14 nanoseconds. Proper selection ofmaterials and geometry for the electro-optic element of the presentinvention will allows switching speeds of 14 nanoseconds or faster, thusfacilitating mid-line, pixel-by-pixel spot position correction.

By incorporating the above described spot position control methodologywith the appropriate apparatus for xerographic printing, including, forexample, a photoreceptor belt or drum, means for moving thephotoreceptor, means for charging the photoreceptor, means for forming alatent image on the photoreceptor, means for transferring the latentimage to paper, means for erasing the latent image from thephotoreceptor and for cleaning the photoreceptor, paper transport means,and means for fusing the image on to the paper, a complete xerographicprint engine and method for improved operation of same may be obtained.Details of the structure and operation of printer devices in general arebeyond the scope of the present disclosure, however they are well knownto those skilled in the art. It will be appreciated from the abovedescription, though, that the present invention is particularly wellsuited for inclusion in those printing applications employing ROS as aportion of the printing process, as well as other printing applications.

In the above discussion it has been assumed that the scanning device(e.g., 26 of FIG. 1) is a rotating polygon having at least onereflective facet (e.g., 28 of FIG. 1). However, certain embodiments ofthe present invention obviate the need for a scanning device. Forexample, in the embodiment 300 shown in FIG. 8, rather than scanning abeam across the image plane, a line-width beam from source 302 ispixel-by-pixel modulated by a modulator 304 and projected to an imageplane 306 by appropriate optics 308. The basic configuration of thisembodiment is similar to that shown and described in U.S. Pat. No.4,638,334 to Burnham et al., dated Jan. 20, 1987, so that details of theworkings of the modulator and related elements are beyond the scope ofthis disclosure. However, in addition to the modulator and relatedelements, the embodiment of FIG. 8 includes the appropriate apparatus310, such as the aforementioned electro-optic semiconductor prism, tofacilitate line position (as opposed to spot position) control in theslow scan direction on the image plane.

Described above are embodiments employing two distinct methods ofmodulation -- direct modulation of the light source and modulation byway of total internal reflection and a zero stop. Other modulationschemes may, however, be employed without departing form the spirit andscope of the present invention. For example, another method ofmodulating beam 16 would be to project it either onto or through amodulator device (not shown), such as an electro-optic or acousto-opticmodulator, etc. Placement of the modulator device along the beam pathwill depend upon its type, the configuration of apparatus 10, etc., aswill be appreciated by one skilled in the art.

In general, to those skilled in the art to which this invention relates,many changes in construction and widely differing embodiments andapplications of the present invention will suggest themselves withoutdeparting from its spirit and scope. For example, the present inventionoperates equally well, and without significant modification, to controlspot position in a single beam ROS or, en block, spot positions in amultiple beam ROS. Furthermore, it is possible to fold the optics of aROS apparatus incorporating the present invention, and thereby compactthe apparatus, by employing a reflective or transmissive/reflectivebody, as opposed to the aforementioned purely transmissive body, as theelectro-optic element. Further still, the apparatus and method of thepresent invention may be combined with other apparatus and/or methods ofcontrolling spot position to achieve advantageous results. For example,the inventions of copending application Ser. Nos. 07/747,166 and07/747,889 may be employed herewith such that bow correction may beaccomplished on individual spots by those inventions, while all spotsemitted form a multiple laser array are selectively positioned by thepresent invention to correct for motion quality errors. Thus, thedisclosures and descriptions herein are illustrative, and are notintended to be in any sense limiting.

What is claimed is:
 1. A method of controlling the slow scan directionposition at which a beam of light is focussed to a spot upon an imageplane in an optical output system, comprising the steps of:directing abeam of light so as to be deflected by an electro-optical element; andapplying an electrical bias to the electro-optic element such that theamount that the beam of light is deflected in the slow scan direction isvaried to achieve slow scan direction position control of the spot. 2.The method of claim 1, wherein the applied bias causes a change in theindex of refraction of the electro-optic element, and the slow scandirection position control of the spot is achieved by varying said indexof refraction.
 3. The method of claim 1, further comprising the step ofvarying the electrical bias applied to the electro-optic element inresponse to predetermined correction data for correction of recurrentslow scan direction position error.
 4. A method of controlling the slowscan direction position of a light beam focused to a spot on an imageplane in a raster output scanning apparatus, comprising the stepsof:generating a light beam from a light source; modulating the lightbeam in response to an image data signal; passing the light beam throughan electro-optical element disposed in the path of the light; focussingthe light beam to form a spot of a desired size and shape on the imageplane; scanning the spot in a raster fashion in a fast scan direction onthe image plane; scanning the spot in a slow scan direction which isnormal to the fast scan direction; determining the existence and extentof error in the slow scan direction position of the spot on the imageplane; and correcting for any slow scan direction position error byvarying the index of refraction of the electro-optic element by varyingits electrical properties in response to the determination of theexistence and extent of such error.
 5. The method of controlling theslow scan direction position of a light beam of claim 4, wherein theindex of refraction of the electro-optic element is varied by varyingthe electrical bias applied to the element.
 6. The method of controllingthe slow scan direction position of a light beam of claim 5, furthercomprising the step of:varying the electrical bias applied to theelectro-optic element in response to predetermined correction data forcorrection of recurrent slow scan direction position error.
 7. A methodof controlling the slow scan direction position of a light beam focusedto a spot on a photoreceptive element in a raster output scanningapparatus, comprising the steps of:generating a light beam from a lightsource; modulating the light beam in response to an image data signal;passing the light beam through an electro-optic element disposed in thepath of focussing the light beam to form a spot of a desired size andshape on the photoreceptive element; scanning the spot in a fast scandirection on the surface of the photoreceptive element; moving thephotoreceptive element in slow scan direction which is normal to thefast scan direction in a phase relationship with the scanning in thefast scan direction; determining the existence and extent of error inthe slow scan direction motion of the photoreceptive element; andcorrecting for any slow scan direction motion error by varying the indexof refraction of the electro-optic element by varying its electricalproperties in response to the determination of the existence and extentof such error.
 8. The method of controlling the slow scan directionposition of a light beam of claim 7, wherein the index of refraction ofthe electro-optic element is varied by varying the electrical biasapplied to the element.
 9. The method of controlling the slow scandirection position of a light beam of claim 8, further comprising thestep of:varying the electrical bias applied to the electro-optic elementin response to predetermined correction data for correction of recurrentslow scan direction position error.